/Engine/Source/Runtime/TimeManagement/Public/FrameNumberNumericInterface.h
To solve this, the Spinboxes will still use double as their type, but double now means frame number and not time. A double can store the entire range of int32 (which backsFFrameNumber) without precision loss, and we can execute callbacks to find out what framerate the sequence is running at. This allows us to display in Timecode, Time or Frames and convert back and forth to double for the UI, and from double into the backingFFrameNumberfor the sequence.
struct FFrameNumberInterface : public [INumericTypeInterface< double >](API\Runtime\Slate\Widgets\Input\INumericTypeInterface)